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You can find other app specific tools, such as the animation rig retargeting tool, under the tools folder in the 3D app python folder. In this case you would be looking into "AtomsMaya/ui/tools".
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If you look in the "AtomsMaya/hostbridge" folder, you'll find few python files. Each one of them is used in different contexts. You will find the same files under "AtomsHoudini/exportershostbridge".
The atomsnode.py contains the host bridge used by the Atoms UI.
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